[linearelliptic...swipdg] updated estimator

Moved ComputeEstimator out of the discretization and replaced local
nonconformity estimator by an appropriate functor.
[linearelliptic...swipdg] Added LocalResidualEstimatorESV2007 functor
[linearelliptic...swipdg] added LocalDiffusiveFluxEstimatorESV2007 functor
[linearelliptic...swipdg] added first draft of SWIPDGEstimator
[linearelliptic...swipdg] finished estimator restructuring
[linearelliptic...swipdg] added ESV2007AlternativeSummation estimator
[test.linearelliptic-swipdg] added effectivity_ESV2007_alt
[test.OS2014] added another aspect of the nonparametric convergence study
[test.linearelliptic-swipdg] fixed available_estimators_()
[test.linearelliptic-swipdg] added expected results/rates